Abstract: For many years, stainless steel composites are widely used in pressure vessels and pressure pipelines because of their good corrosion resistance and low cost. According to researches, when welding cannot be performed from the inner wall of the small-diameter stainless steel composite pipes, a minority of construction companies first use 18-8 stainless steel weld material as the backing weld; then 25-13 weld material is used to weld transition layer; at last, conventional carbon steel or low alloy steel weld material is used to weld the base layer. This welding sequence is not implemented in accordance with the provisions of the relevant specification: "The welding of the base layer should be the same as the welding of the transition layer." This kind of "non-standard" welding process is simple with low cost since the 25-13 stainless steel welding material is replaced by carbon steel welding material. And none quality problems are found during the running process of the pipelines. This paper describes the process of the comparative welding tests on the welding procedures of several welding sequences of stainless steel composites and the mechanical properties test and hardness test of the welded joints. Then it analyzes the results.

Key words: stainless steel composites, welding sequence, martensite, hardness